1. Field of the Invention
The invention relates generally to an apparatus for measuring flow rates in a fluid, such as in oil and gas wells.
2. Background Art
Venturi flow meters are commonly used to measure flow rates in multiphase fluids in a well bore, such as oil, gas, and water produced from hydrocarbon underground reservoirs. An example of such a flow meter is disclosed in U.S. Pat. No. 4,856,344. The basic premise of a Venturi flow meter is to introduce a restriction in the flow path. The restriction is commonly referred to as the “throat.” Fluid pressure is measured in the throat and at a location upstream of that restriction. Because the flow area in the throat is smaller than the upstream portion, the velocity of the fluid increases, which results in a corresponding decrease in pressure in the throat, according to the well known Bernoulli principle according to which the total energy in a steadily flowing fluid system is a constant along a flow path. By comparing the two pressure readings and the respective cross-sectional areas, a flow rate can be determined using the relationship between fluid velocity and pressure.
FIG. 1 shows a basic layout of a typical Venturi flow meter 100 disposed within a pipe 101 that can be used in a well bore. The upward flow direction of the fluid through the flow meter is shown by arrow F. The primary portions are an upstream section 120 and a throat section 110. A convergent tapered section 115 connects the upstream section 120 and the throat section 110. The convergent tapered section 115 provides a smooth reduction in diameter between the larger upstream section 120 and the smaller throat section 110 in order to minimize turbulence in the flow meter. Downstream of the throat section 110, a divergent tapered section 130 provides a smooth transition to the larger downstream section 140, which reduces the fluid velocity and minimizes turbulence and associated pressure losses. A pressure gauge P3 measures the fluid pressure in the throat section 110 where the diameter is D2. Another pressure gauge P2 measures the fluid pressure in the upstream section 120 at a point upstream of the convergent tapered section 115 at a point where the pipe diameter is D1. The ratio of the diameter D2 over the diameter D1 is known as the Beta ratio of the Venturi flow meter and is generally about 0.5. Taking into account the Beta ratio, a comparison between the two pressure gauges P2 and P3 provides a differential pressure, which allows for the determination of the flow rate of the fluid based on well known equations.
In addition to flow rate measurements, a typical Venturi flow meter may further include a gradiomanometer to determine the density of a multiphase fluid. To determine the average density of the multiphase fluid, a differential pressure is taken between two pressure gauges P1 and P2 located in the upstream section 120 of the flow meter 100 and separated by a distance H1. The differential pressure correlates with the average density of the multiphase fluid between the two pressure gauges P1 and P2.
In a typical oil well, inside the wellbore where the hydrocarbon fluid is flowing, the fluid flow rate may vary significantly, especially during transient periods of testing and production. In particular, when carrying out drill stem testing (DST), which is used to evaluate the production potential of hydrocarbon reservoirs, the fluid flow rate may be lower than expected or vary widely. Thus, the pressure differential between the throat section and the upstream section may be too small so that accurate flow rate measurement results may not be obtained. In order to increase the pressure differential and thus to be able to accurately measure small flow rates, the cross-sectional area of the throat section is made to be very small. However, if the cross-sectional area of the throat section is very small, various downhole tools will not pass through the Venturi flow meter. For example, production logging tools and firing guns deployed by wireline may have a maximum diameter of over 5 cm (2 in.), making it impossible for those tools to pass through the small throat section. This is one of the reasons why conventional flow meters have not been used successfully downhole in a wellbore, and typically, the flow rate has to be measured at the surface.